1. Field of the Invention
This invention relates to a flat cable conductor with a solder plating film formed on the surface, method of making the flat cable conductor and a flat cable using the flat cable conductor.
2. Description of the Related Art
At present, a flexible flat cable (herein referred to as FFC) is used as an internal wiring member for external storage devices (CD-ROM drive, DVD-ROM drive etc.) in a small printer, a personal computer etc. and for electronic products such as a video game machine.
FIG. 1 is a perspective view showing a basic structure of a conventional FFC. The FFC 30 is a thin and flexible tape-shaped cable or wire with a thickness of 100 to 300 μm , where a plurality of flat cable conductors 32 with a thickness of several tens of micrometers are in parallel arranged into a conductor array. The FFC 30 is manufactured such that the conductor array of flat cable conductors 32 arranged in parallel is sandwiched by insulative plastic films 33a, 33b with an adhesive layer formed on one surface thereof, and it is continuously pressure-bonded by a hot roll. A solder plating rectangular soft copper wire is used as the rectangular conductor 32, nonflammable polyester is used as the adhesive, and a polyethylene terephthalate film is used as the insulative plastic films 33a, 33b (Hiroshi Yamanobe et al., “Reliability of Flexible Flat Cable (FFC)”, Hitachi Cable Review, Hitachi Cable, Ltd., 2000, No. 19(2000-1), p. 53–56).
The flat cable conductor 32 is, as described above, made of the solder plating rectangular soft copper wire and it is manufactured, for example, as described below. First, a solder (Sn-5 wt % Pb) plating film is about 6 μm thick electroplated around a pure copper wire (φ0.6 mm) as a base material. Then, the solder plating pure copper wire is cold-drawn to φ0.1 mm and then it is rolled into a precise rectangular wire with predetermined dimensions (e.g., 0.05 mm in thickness and 0.32 mm width). Finally, the precise rectangular wire is annealed to have the solder plating rectangular soft copper wire (Japanese patent No. 2942458).
In recent years, from the viewpoint of environmental suitability, a Pb-free material (non lead material) or non-halogen material is desired, and therefore a corresponding solution is also required of materials used for the FFC.
Thus, it is demanded that the solder plating film on the surface of the flat cable conductor 32 is free from Pb.
The solder plating film used thus far is generally made of a solder that 1 to 10 wt % lead (Pb) is added to tin (Sn). Pb is added to Sn so as to reduce the melting point of Sn plating and to prevent the generation of whisker (hair-like single crystal) from the surface of the pure Sn plating film. Therefore, when Pb is removed from the conventional solder composition to have the pure Sn plating so as to address the Pb-free solution, an increase in the melting point and the generation of whisker will be of concern. The generation of whisker is not desirable since it causes a short-cut between the conductors of FFC.
So, as a solder Sn alloy used for Pb-free solder plating, for example, Sn—Bi alloy, Sn—Ag alloy and Sn—Cu alloy are developed, and corresponding electroplating solutions are developed and are partially in practical use. The Sn alloys for solder plating need to have reliability properties, such as melting point, solder wettability and anti-whisker property, equal to those of the conventional ones Some FFC's using such a Pb-free solder plating rectangular conductor are disclosed (Japanese patent application laid-open Nos. 2001-43743, 2001-43744, and 2001-43745).
Further, some FFC's are disclosed that its solder plating film of pure Sn is thermally refined to prevent the generation of whisker while keeping its solder wettability (Japanese patent application laid-open Nos. 2001-73186, 2000-173364 and 2002-42556).
However, the Pb-free solder plating is inferior to the conventional Sn—Pb plating in the aspect of easiness of electroplating and manufacturing cost Therefore, a further improvement is desired thereon.
The FFC using the Pb-free solder plating rectangular conductor has, in itself, a high reliability in anti-whisker property. However, as a metal terminal plating to be fitted to a FFC is recently made to be Pb-free, its compression stress is likely to increase when the FFC is fitted or connected to the connector. Along with the increase of compression stress, a whisker is likely to be generated from the surface of the Pb-free solder plating film in the flat cable conductor of PFC. Therefore, a further enhancement in reliability of the Pb-free solder plating is desired,